Material and method for removing coatings of nickel or the like from a metal base



Patented May 14, 1940 UNITED STA-r15:

MATEBIAL AND-METHOD FOR REMOVING COATINGS F NICKEL OR THE LIKE mom aMETAL pass A Earl F."Blll'di0k ern Electric Company,

Chicago, 111., assignor to West- Incorporated. New York,

N. Y., a corporation of New York No Drawing. Application May 10, 1939,

' Serial No. 272,755

. 9 Olaims. (01. 148-5) This invention relates to materials and methodsfor removing coatings oiv nickel or the like from a metal base.

An object of this invention is the provision of an improved andinexpensive solvent that will -rapidly remove a coating of nickel orsimilar material from a ferrous base without attacking the base, andmethodsjor using the solvent emciently.

Many metal products, especially articles made of iron, steel or otherferrous base materials, are provided with an ornamental or protectivecoating of nickel which is usually applied by electroplating. Often theweight of the nickel coating is critical and in most cases an undulyheavy coating is unnecessarily expensive. For these reasons it is nearlyalways important to apply a coating that lies within a specified weightrange. To permit efiective control of the electroplating process, andconsequently of the coating weight, a rapid and accurate coating weightdetermination method is required. To make this determination, it isdesirable to remove the coatin completely and selectively so that theweight of the article before and after stripping will quickly disclosethe coating weight.

Solvents have been used-for stripping various coating and base metalcombinations, but the chemical properties of nickel and iron are sosimilar that it is diihcult to effect a separation of these twoparticular metals. As a result, the prior methods for stripping nickelfrom iron have been very slow, or it has been necessary to sacriliceaccuracy to gain speed by using agents that tend to attack the ferrousbase.

In one prior method, the nickel was removed by immersing the coatedarticle in concentrated nitric acid. An accurate result is availablewith this method, but the action of the acid is very slow. It requiresseveral hours to dissolve a standard 50 milligrams per square inchnickel coating in nitric acid, which correspondingly delays anyadjustment of the platingprocedure' that is indicated by a coatingdiscrepancy.

Because iron precedes nickel in the electromo-. tive series, diluteacids are not very suitable for this purpose without the additionof"some selective inhibitor for the iron. Nitric acid is one of the bestacid solvents for nickel, but it is an oxidizing agent and most of theeffective inhibitors are organic compounds which are not compatible withoxidizing agents. Alkaline solutions, such as ammonium hydroxide plusbromine or. hydrogen peroxide, are unsatisfactory becaus their action isnot sufliciently rapid.

Separation of these metals on the basis of differences in their physicalproperties is not very practical. The difference in the expansioncoefllcient of nickel and iron is not sufficiently great to cause theplate to crack off when the article is heated in air and heating inhydrogen or molten sodium cyanide or charcoal does not facilitateremoval of the plate.

Experience with these various prior methods has emphasized the need fora procedure that will measure the nickel coating rapidly and accuratelyand thus provide the plating process for regulating the weight of thecoating as the coating is being applied. In

accordance with one embodiment of this invention a coating of nickelhaving a weight around 50 milligrams per square inch is removedcompletely and accurately from a ferrous base in less than five minutesby immersing the plated article in concentrated nitric acid and a smallquantity of chlorine.

The eifectivechlorine content of the solvent mixture is important andthe proper quantity of chlorine is supplied by adding a compound ormaterial that is a source of chlorine in concentrated nitric acid.Suitable addition agents are sodium chloride, potassium chloride orhydrochloric acid. If sodium chloride is used, .056 gram of the chlorideis added for every 100 cubic centimeters of nitric acid having aspecific gravity of 1.40-1.42 at60" F. This particular concentrationgives the best results but the proportion of sodium chloride can varybetween .03 and .10 gram per 100 cubic centimeters of nitric acid toprovide proportionately eifective solutions.

If hydrochloric acid is employed, the best results are obtained with amixture containing .08 part of concentrated hydrochloric acid (Sp. Gr.1.18-1.19 at 60 F.) and 100 parts of concentrated nitric acid (Sp. Gr.1.40-1.42 at 60 F.) by volume. By reducing this hydrochloric acidconcentration to .04 part of hydrochloric per 100 parts of nitric acid,which afiects the total solution volume very slightly, the process issubstantially retarded but the reduced speed is still much greater thanthe speed available with nitric acid or other prior agents. An increaseinthe hydrochloric acid to .20 part of hydrochloric per-100 parts ofnitric acid by volume accelerates the solvent action somewhat, but theupper tionable attack of the ferrous base occurs.

Other chlorine compounds that are soluble in the nitric acid and whichdo not contain an element that will inhibit or impair the limit of thisincreased concentration approaches a range where objecsolvent action anefiecti ve control of can be used. If some'materi'alothi than hydrochloric acid or. sodium chlorideis used, a; quantity that will supply anequivalent proportion of chlorine to the above described ratiosis addedto the nitricacid. 1"

Assoon-as the coated part is immersed in the solution, violent gassingbegins. The gas generation continues for a fewminutes or until thenickel is completely "dissolved and-then ceases abruptly. Theferrousbase is not appreciably attacked by the solvent and the solvent alsoprotects a base containing an appreciable proportion ofnickel, such asmagnetic alloys of the Permalloy" type. The solvent has been used forstripping nickel plate from Permalloy containing 45% nickel and thebalance iron, and removed the nickel without attacking the basesufliciently to effect the accuracy of the coating weight determination.

With this method a nickel coating of around fifty milligrams per squareinch of base surface. which is used quite generally, can be removed inthree and one-half minutes. All of the coating is dissolved during thisinterval and the coating weight is determined by merely weighing thebase before and after the stripping process. Because this determinationis made very rapidly, as well as accurately, a close control oftheelectroplating procedure is possible to minimize the production ofdefective articles.

The solvent is also useful for stripping nickel coatings from'ferrousbase articles in cases where replating of defective or used articles isnecessary. Replating of large quantities of parts is made economical byrapid removal of the old coating in a process which preserves thedimensions of the base and also avoids the necessity for grinding ormachining the base before the new coating is applied.

The solvent is also used in connection with microscopic examinations ofnickel coatings. A quick dip oi. the plated surface in the solventproduces a surface etch which reveals the crystal structure of the plateeifectively.

It will be apparent that other uses and adaptations of the invention arefeasible and it is to be understood that the invention is limited onlyby the scope of the appended claims.

What is claimed is:

1. A solvent for removing a coating of nickel from an iron bearing basecomprising concentrated nitric acid, and a chlorine compound that issoluble in the nitric acid, the chlorine compound being added to thenitric acid in sufllcient quantities to establish the same nitric aeidchlorine ratio that is provided when from .03 to .10 gram of sodiumchloride is dissolved in 100 cubic centimeters of concentrated nitricacid.

2. A solvent for removing a coating of nickel from..a ferrous metal basecomprising 100 cubic centimeters of concentrated nitric acid, and from..03 to. .10 gram of sodium chloride dissolved in the nitric acid.

3. A solvent for removing a coating of nickel from a ferrous metal basecomprising 100 cubic centimeters of nitric acid having a specificgravity of 1.40-1.42 at 60 F., and about .056'gram of sodium chloridedissolved in the nitric acid.

4. A solvent for removing a coating of nickel from a ferrous compositionbase comprising 100 parts by volume of concentrated nitric acid, andfrom .04 to .20 part by volume of concentrated hydrochloricacid.

5. A solvent for removing nickel from a ferrous metal base comprising100 parts by volume of nitric acid having a specific gravity of 1.40-142at 60 F., and .08 part by volume of hydrochloric acid having a specificgravity of 1.18-1.19 at 60 F.

6. A method of stripping a coating of nickel froman iron bearing basewhich comprises the step of immersing the coated base in a combinationof concentrated nitric acid anda chlorine compoundthat is soluble in thenitric acid, the compound being added in quantities that will supply thesame proportion of chlorine that is provided when from .04 to .20 partby volume of concentrated hydrochloric acid is added to 100 parts byvolume of concentrated nitric acid.

7. A method of stripping a coating of nickel from a ferrous metal basewhich comprises the step of immersing the coated base in a mixture of100 parts by volumeof nitric acid having a specific gravity around 1.41at 60 F., and .08 part by volume of hydrochoric acid having a specificgravity around 1.18 at 60 F.

,8. A method of removing a coating of nickel from a ferrous metal basewhich comprises the step of immersing the coated base in a solventsolution of 100 cubic centimeters of concentrated nitric acid and about.056 gram of sodium chloride..

9. A method of removing a coating of nickel from a ferrous compositionbase which comprises the step of immersing the coated base in a solventsolution of nitric acid and sodium chloride in the proportion of .03 to.l0 gram of sodium chloride to 100 cubic centimeters of concentratednitric acid.

EARL F. BURD'ICK.

DU quauvau-v -ww...

CERTIFICATE OF CORRECTION. Patent. No.'2,2oo,h 6. May 11;, 191m.

' EARL F. BURDICK.

"It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,seeand colunmflineZl, claim re ute-1&2" read- --l.h.O-l.LL2-; and thatthe said Letters" Patent should be read with this correction thereinthat I the same may conform to the record of the case in the PatentOffice.

I Signed and. sealed. this 30th day of July, A. D. 191m;

4 Henry Van Arsda'le;

(Seal) Acting Corhmissipner 'of Patents;

